Method and apparatus for handling of covering gas

ABSTRACT

A method for the handling of covering gas or inert gas in connection with a tank that conveys crude oil, during successive discharging and loading operations of the tank, is disclosed. The method is characterised by the fact that covering gas/inert gas is re-circulated in an approximate closed circulation by the following steps: that during discharging of the tank, covering gas is produced in a recognised manner known per se from the cargo to fill the gradually increasing void space in the tank, and that when the tank is filled again with crude oil, the gas from the tank is extracted again, and it is compressed and absorbed into the stabilized crude oil. An apparatus is mentioned for accomplishing the method.

[0001] The invention presented here concerns a method and an apparatus for handling covering gas as stated in the introduction to the following patent claims 1 and 9.

[0002] The loading and carriage of, for example, crude oil by vessel, leads to large emissions of cargo components such as hydrocarbon gas (HC) from oil cargoes, also termed VOC.

[0003] Today some vessels are equipped with an inert gas generator that burns oil/diesel. The exhaust that is cleaned with seawater will mainly consist of nitrogen, carbon dioxide and a small residue of oxygen.

[0004] Since there may be residues of flammable gases from the cargo in the tanks, the ingress of air could create an explosive mixtures which makes a safety hazard. As the cargo is delivered, the empty volume must be replenished with inert gas, to prevent oxygenated air from entering the tanks.

[0005] One problem with current methods is that the inert gas used for making the tank atmosphere safe leads to a higher level of vaporization of the cargo than necessary, plus the fact that inert gas is corrosive.

[0006] To procure another type of inert gas at various terminals will create a logistics problem since many sites will not have access to such gas. Further, there will be a need for converting vessels and terminals with connections and hoses for supplying a protective atmosphere to the tanks.

[0007] According to another aspect of the invention, which is stated under the following claim 9, an apparatus is defined that implies that the covering gas can now be re-circulated in a virtually closed circuit with hardly any emissions to air, and as follows:

[0008] 1. During discharge of the tank, covering gas is produced, as stated in the foregoing, (i.e. from the actual cargo/crude oil) in order to fill the gradually increasing void space in the tank.

[0009] 2. When the tank is filled once more with cargo, the tank gas is vented again, then compressed and absorbed in the cargo/crude oil.

[0010] The crude oil will act as a buffer tank where the gas will be taken out on demand and fed back when the gas volume diminishes in the cargo tanks.

[0011] The particular asset of the apparatus according to the invention is characterised by the features presented in claims 10 and 11.

[0012] With this invention, an explosive atmosphere is avoided by preventing the ingress of oxygen into the tank space. Accordingly, a gas mixture that has been extracted from the cargo is utilised as a sort of covering gas and not as a propellant gas. As and when the cargo is delivered, the void space must be replenished so that oxygenated air shall not enter into the tanks. Extracting the gas continually from the actual cargo now solves this matter.

[0013] One advantage of this new covering gas is also that the corrosion is reduced, since inert gas is not used with carbon dioxide and salt, which combine to form a highly corrosive atmosphere.

[0014] One example of the proposed method is based on combining such a covering gas producing plant with a facility for recovering covering gas by absorption.

[0015] When the vessel comes to the discharging terminal, a generator will recover covering gas from the cargo. This gas is used for replenishing the void space in the tank. After the vessel has delivered its cargo, the empty tanks will be protected by overpressure of gas that has been extracted from the cargo.

[0016] When the vessel comes to the loading buoy or production ship, the cargo will be filled up in the empty tanks. All covering gas will go to the absorption plant that will extract the gas and supply it to the new cargo. The covering gas is taken out again when discharging is done, thus forming a re-circulating system.

[0017] According to the invention, a gas generator can be designed in two ways. One can use either heat or vacuum to release the gas. A combination of methods is also possible. To increase vaporisation, one can let the crude oil occupy a wide surface by atomising it into droplets or by letting it circulate (run) over large areas. This can be produced by conveying some of the cargo into a combination tower that is used for absorption during recovery and degassing during discharging.

[0018] Another possibility is to use a nozzle system where the cargo is atomised by being sprayed into the tank under pressure. The cargo will emit gas to be used as covering gas as and when the tanks are discharged.

[0019] The following account gives a description of a gas generator that is based on heat. To reduce energy consumption with the method according to the invention, the hot cargo can be directed through a heat exchanger that preheats the cargo into the generator. By continually monitoring tank pressure and the amount discharged, one can control the amount directed through the gas generator, and consequently, one can regulate the production of gas that is conveyed back to the tank as covering gas.

[0020] The gas atmosphere, thus, will circulate by virtue of the fact that it is released at the terminal and combines with the cargo during the loading.

[0021] The example mentioned is detailed in the following description referring to the accompanying drawing, which shows a typical design of a gas generator for covering gas according to the invention.

[0022] The FIGURE shows a tank (10 that is partly filled with a fluid 12, as the volume above the fluid level 14 comprises a covering gas 16. A line 18 is used to supply load and remove discharge the tank 10 of crude oil. From a branching point 20 on the line 18 runs a line 22 via a pump 24 further through a heat exchanger 26, and a heat exchanger 28 to the upper part of the column 30. From the top of the column 30 runs a line 32 back to the top of the tank 10.

Use of the Apparatus According to the Invention

[0023] Discharging of the Tank 10

[0024] The FIGURE depicts a tank 10 that is about to be discharged through the line 18. By means of the pump 24 a small part of the cargo is extracted in the line 22, and passes through the heat exchanger 26 and the heat exchanger 28 whereupon a part of the extracted cargo vaporizes. Additional vaporization is obtained by means of spraying it into the top of the column 30 to attain further vaporization. In gas form the covering gas is then conducted to the tank top and the remaining cargo is cooled and returned.

[0025] Normally, only a part of the extracted fluid will vaporize, and the rest of the liquid cargo will run back to the cargo through the line 36.

[0026] Loading of the Tank 10

[0027] In this situation the tank 10 is filled with liquid. The filling of the cargo is done through the line 18. The gas is now recovered through a standard absorption plant where the absorption column could be the same as the degassing column during discharge.

[0028] During the filling of the tank, the column 30 is used to absorb covering gas that is drawn off from the tank, since the covering gas is pressurized and absorbed in a cargo fluid that is led through the column 30. 

1. Method for the handling of covering gas or inert gas in connection with a tank that conveys crude oil, during successive discharging and loading operations of the tank, characterised in that the covering gas/inert gas is re-circulated in virtually closed circulation by the following steps: that during discharging of the tank covering gas is produced in a recognised manner known per se, from the cargo to fill the gradually increasing void space in the tank, and that when the tank is filled again with crude oil, the gas from the tank is extracted again, and it is compressed and absorbed into the stabilized crude oil.
 2. Method according to claim 1 characterised in that the covering gas is produced from the cargo containing components that can be vaporized or degassed.
 3. Method according to claim 1-2, characterised in that the covering gas is produced simultaneously with the tank being emptied of cargo, or when changes in pressure occur in the tank, so that a slight overpressure in the tank is maintained.
 4. Method according to any of the foregoing claims, characterised in that a part of the cargo is extracted and conducted through a plant, such as a gas generator for producing gas, said gas being pressurized and fed back to the tank.
 5. Method according to any of the foregoing claims, characterised in that preheating the part boosts the vaporization of part of the cargo, subjecting it to vacuum and/or atomisation.
 6. Method according to claim 5, characterised in that preheating is carried out by means of a part of the cargo being led through a heat exchanger and/or over a heating element.
 7. Method according to claim 5, characterised in that a vacuum is used in the gas generator.
 8. Method according to claim 5, characterised in that a column is used for atomisation, where the unit of cargo is sprayed in at the top to form droplets.
 9. Apparatus for the handling of covering gas or inert gas in connection with a tank conveying crude oil, during successive discharging and loading operations of the tank, characterised by a circulation for re-circulating covering gas/inert gas, comprising: means of vaporization/vaporization of a unit of cargo for the production of said covering gas, and means for filling a gradually increasing void space in the tank with covering gas, and an means for venting gas from the tank, as well as a means of compressing and absorption of the released tank gas.
 10. Apparatus according to claim 9, characterised by a cargo tank (10), a loading and discharge line (18), a branch pipe (22) with pump (22) for the venting of a unit of cargo, means (24,28) for tempering the unit of cargo, a column (30) for atomisation/vaporization of the cargo unit, in that a first stage line (32) connects the top of the column (30) to the cargo tank (10) and a second line (36) connects the base of the column (30) to the cargo tank (10).
 11. Apparatus according to any one of claims 9-10, characterised in that the column (30) in the circulation is used both for vaporization/vaporization of the cargo unit and for (later) absorbing pressurized covering gas, which is drawn off from the tank, in a cargo fluid that is directed through the column (30). 